На этот вопрос уже есть ответ здесь:
$echo red | grep -q red
$echo $?
0
'grep -q' отключает запись в стандартный вывод. $? статус выхода равен 0, если совпадение найдено, иначе не 0. Это в значительной степени объясняет приведенный выше код. Я хочу узнать больше о «$?». Что это такое ??
От Special Parameters
раздел man bash
:
Special Parameters
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When
the expansion is not within double quotes, each positional
parameter expands to a separate word. In contexts where it is
performed, those words are subject to further word splitting and
pathname expansion. When the expansion occurs within double
quotes, it expands to a single word with the value of each
parameter separated by the first character of the IFS special
variable. That is, "$*" is equivalent to "$1c$2c...", where c
is the first character of the value of the IFS variable. If IFS
is unset, the parameters are separated by spaces. If IFS is
null, the parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from one. When
the expansion occurs within double quotes, each parameter
expands to a separate word. That is, "$@" is equivalent to "$1"
"$2" ... If the double-quoted expansion occurs within a word,
the expansion of the first parameter is joined with the begin‐
ning part of the original word, and the expansion of the last
parameter is joined with the last part of the original word.
When there are no positional parameters, "$@" and $@ expand to
nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the exit status of the most recently executed fore‐
ground pipeline.
- Expands to the current option flags as specified upon invoca‐
tion, by the set builtin command, or those set by the shell
itself (such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the sub‐
shell.
! Expands to the process ID of the job most recently placed into
the background, whether executed as an asynchronous command or
using the bg builtin (see JOB CONTROL below).
0 Expands to the name of the shell or shell script. This is set
at shell initialization. If bash is invoked with a file of com‐
mands, $0 is set to the name of that file. If bash is started
with the -c option, then $0 is set to the first argument after
the string to be executed, if one is present. Otherwise, it is
set to the filename used to invoke bash, as given by argument
zero.
_ At shell startup, set to the absolute pathname used to invoke
the shell or shell script being executed as passed in the envi‐
ronment or argument list. Subsequently, expands to the last
argument to the previous command, after expansion. Also set to
the full pathname used to invoke each command executed and
placed in the environment exported to that command. When check‐
ing mail, this parameter holds the name of the mail file cur‐
rently being checked.
Обратите внимание, что "последний раз выполняемый приоритетный конвейер" является не всегда последней командой (если промежуточная команда перестала работать, например); в bash
, можно получить доступ к статусу выхода каждой команды отдельно в канале через PIPESTATUS
выстройте при необходимости в информации о статусе выхода с более прекрасными зернами:
PIPESTATUS
An array variable (see Arrays below) containing a list of exit
status values from the processes in the most-recently-executed
foreground pipeline (which may contain only a single command).
Напр.
$ echo 'foo bar' | grep 'foo' | sed 's/f/g/'
goo bar
$ echo "${PIPESTATUS[*]}"
0 0 0
$ echo 'foo bar' | grep 'goo' | sed 's/g/f/'
$ echo "${PIPESTATUS[*]}"
0 1 0
Переменная $?
в bash хранит код выхода последней выполненной команды. В вашем примере это будет означать, что echo red | grep -q red
завершился с кодом 0
, который почти всегда является признаком успешной команды.
Вы можете проверить это с различными другими командами, чтобы увидеть их коды возврата. Например,
commandthatdoesntexist; echo $?
вернет код ошибки 127
и т. Д.